1. Field of the Invention
The invention relates to polymeric pipes reinforced by a metal framework, to a method for producing and optimal variants of using them. Both the strength of a metal framework and the chemical stability of a polymeric matrix enable to use metal-polymeric (metal-plastic) pipes in various fields of economy, in particular, for transportation of oil and gas, acids, alkaline products, drinking and process waters, and the high stability against abrasive wear enables to use them for transportation of corrosive and neutral pulps and as case pipes, e.g., in a case of underground leaching of rocks.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
A metal-polymeric reinforced pipe is known that comprises a welded metal framework and a polymeric matrix; it is described in USSR Inventor's Certificate No. SU1366757 published on Jan. 15, 1988. A drawback of this pipe is a complex combined structure of the polymeric matrix that is produced in two steps. First, the inner layer of the polymeric matrix is formed, a metal framework is embedded into it, and then the upper, coating thermoplastic layer is formed, the inner layer and the outer layer of the pipe being provided with a complex surface profile in order the profiles may engage with, and the layers may adhere to, each other.
A metal-polymeric reinforced pipe that is known from GB Patent No. GB2277975, which is published on Nov. 16, 1994, is simpler and more durable. This pipe has a monolith, thermoplastic polymeric matrix having, primarily, a crystalline structure and being produced by extrusion, and a reinforcing metal framework made of longitudinal reinforcing elements and at least two transverse spiral reinforcing elements.
A drawback of this pipe is its low long-term strength at thermal cyclic loads.
The closest analogous solution to the claimed invention is a metal-polymeric reinforced pipe that is described in USSR Inventor's Certificate No. SU929951, which is published on May 23, 1982, and that comprises: a metal net framework rigidly fixed by welding in crossing points of reinforcing elements, and a polymeric matrix produced by extrusion molding. Further, in order to improve resistance to radial loads, the thicknesses of the inner wall and the outer wall are selected in the range from 0.2 to 0.8 of the framework thickness.
However, as ascertained in practice, the strength of a metal-polymeric reinforced pipe, in particular its ability to resist radial loads, is determined mainly not by a ratio of the reinforcing framework thickness and the pipe wall thickness, but, rather, by the ability of the “metal-polymer” pair to relax inner stresses arising under the action of loads on a pipe, which enables to maintain the integrity of a polymeric matrix without allowing cracking the pipe body.
A metal framework and a polymer are heated to the same temperature in the process of pipe production. During their subsequent cooling with equal (close value) gradients, both the metal and the polymer shrink, but shrinkage of the polymer is greater in per cent. Thus, after cooling, a gap remains between the metal framework and the polymer matrix, which gap allows the structure elements to be in a mutually balanced state, that is, in particular, the polymer allows elastic deformation of the framework when the latter is acted on by loads, thus relaxing arising stresses. Furthermore, such a gap allows long relaxation of the polymer without forming large spherulites. In this connection, the more is an amount of a polymer in a cross-section of a pipe, the lower is the polymer relative ability to relax arising stresses. That is, an increase in the inner layer and/or the outer layer of a polymer, as Inventor's Certificate No. SU929951 states, does not allow to improve strength properties of a pipe.
On the other side, a significant increase in a thickness of a polymer layer on a pipe framework is unacceptable, since a definite minimum dimension of the polymer matrix body is required for process purposes, for example, for assembling a pipeline or a pipe string, as well as for obtaining the possibility of repairing a pipeline.
Moreover, in addition to critical observations in respect of the invention under SU929951, it may be stated that it is established, on the basis of experimental works carried out, that the radial strength of a metal-polymeric pipe is mainly determined by the physical properties and parameters of a metal framework, such as: a framework unit cell dimension, dimensions of longitudinal and transverse reinforcing elements, or the strength of a welded connection, rather than selection of a pipe wall thickness.
In the method of producing the claimed metal-polymeric pipe significant attention was paid exactly to improving the strength of the reinforcing framework for a metal-polymeric pipe, which enabled to eliminate problems existing in the production of high-quality pipes, which problems were not removed in the existing state of the art.
A method for continuous making of a reinforced polymeric pipe and a device therefor are known in the art, which are described in Patent SU1716963 published on Feb. 29, 1992. This method comprises feed of a polymer melt into an annular mold cavity of an extruder with the simultaneous feed a reinforcing framework into it. For the purpose of reducing residual internal stresses in the pipe wall an angle between polymer feed and a direction of framework movement is selected in the range from 90° to 150°. The device comprises an extruder having a head with a central feeding passage for a melt. The annular mold cavity used for forming a pipe is composed from a mandrel and a barrel and communicates with the extrusion passage. The outlet region of the extrusion passage is made with a space angle in the range from 60° to 180°, the vertex of this angle facing the exit of the mold cavity.
The reasons for residual internal stresses arising in the wall of a metal-polymer pipe are, in particular, friction forces arising between an extrudate and the extrusion passage walls, and, after leaving an extrusion head, friction forces arising between an extrudate and the mold cavity walls with subsequent fixation of the polymer stressed state during curing in the result of the produced pipe cooling. This stressed state at the macrostructural level is characterized by the longitudinal orientation of the polymer macromolecules that is most prominent in the areas adjacent to a mandrel. It was supposed that the making of the extrusion passage outlet region with a space angle in the range from 60° to 180°, which vertex faced the exit of the mold cavity, would contribute to violation of a polymer melt laminar flow within the extrusion passage, since a melted polymer flow, which experienced a sharp turn when exiting the extrusion passage and entering the mold cavity, created violations in the polymer macrostructural orientation formed in the extrusion passage, and a subsequent orientation of the polymer macromolecules in the mold cavity began with the polymer disoriented state. Due to the fact that time is required for the macrostructural orientation process which is commensurate to the time of polymer passing through the mold cavity, it was supposed that stresses within a material by the time of curing would develop to a lesser degree.
The said supposition did not prove its value, and it was established in practice that the longitudinal orientation of polymer molecules occurred irrespective of an angle of leaving an extrusion passage, since before the beginning of the crystallization time orientation processes in the polymer melt structure are equilibrium. Therefore, no changes in angles and melt movement directions within an extrusion passage makes significant alterations in the process of spatial orientation of molecules in the polymer macrostructure.
A drawback of this technology is a non-optimal structure of a polymer matrix, which causes low indices of long-term strength of a pipe. Long-term strength is estimated by a method of thermal cyclic loading (thermal cycling) of specimens by cooling them in each cycle and keeping them for 3 hours at −40° C. with subsequently heating them to +80° C. and keeping for 3 hours. A number of cycles before destruction begins is, according to the specification to SU1716963, from 130 to 245.
The closest analogous solution to the claimed invention in respect of the method and the device for producing a metal-polymeric reinforced pipe are the method and the device disclosed in Patent No. RU 2319886 published on Mar. 20, 2008. This Patent teaches a method for continuous making of a metal-polymeric pipe by way of extrusion molding, according to which a reinforcing coil is winded with a pre-determined pitch onto longitudinal reinforcement elements strained and moved together with an extruded pipe, said elements being evenly distributed over the circumference, then, while being winded, it is welded to in succession crossed elements of longitudinal reinforcement by the electrocontact method with the use of a roll electrode that is rotated around the axis of a reinforcing coil. Welding current pulses are fed synchronously with times of crossing elements of longitudinal reinforcement. The formed reinforcing framework is introduced into a mold cavity, simultaneously feeding a melt of an extruded polymer into it. Reinforcing coils are arranged with mutual phase shift by an angle of 2 π/n, where n is a number of reinforcing coils. Welding is carried out by simultaneously using several pairs of roll electrodes, the number of which corresponds to a number of reinforcing coil pairs. The welding current is fed to each pair of roll electrodes autonomously. Thus, a number of roll electrode pairs is n/2, where n is a number of reinforcing coils. The central angle α for each electrode pair, that is measured between radii drawn between contact points of electrodes and longitudinal reinforcing coils is 120-240°. And it should be noted that welding current is fed to each pair of roll electrodes via its pair of current-feeding collectors alternatively.
The device for carrying out the method according to Patent No. RU 2319886 comprises an extruder with a direct-flow head which is provided with an arbor having guiding slots for longitudinal reinforcement and a cooled mandrel. A welding machine provided with a drum installed on bearings is arranged downstream the extruder. The drum is provided with a reel for transverse reinforcement with the possibility of freely rotation, a deflecting roll for winding a reinforcing coil, roll electrodes for welding it to longitudinal reinforcement elements, and a current-feeding collector with isolated sections in a number equal to that of roll electrodes. A barrel is fixedly arranged within the drum, which forms a mold cavity together with a mandrel.
The number of roll electrode pairs is n/2, where n is the number of reinforcing coils, and the central angle α in each pair between the contact points between the electrodes and the transverse reinforcing coils is 120-240°. Each roll electrode of a pair is mounted onto a lever that has an eccentric rotation support. Levers on one side of eccentric rotation supports in each pair have counterbalances and are connected therebetween on the other side by a pneumatic cylinder with an indicator and a regulator of welding force. Roll welding electrodes in each pair are connected to each other, to the arbor and to a current source in series.
Drawbacks of the above-described method and device are low strength of welded connections of reinforcing metal framework of a pipe, which is obtained with the use of them, since the force of pressing the welding roll electrodes is provided by a pneumatic actuator that exerts less power than a hydraulic actuator. Also, a drawback of the welding mechanism structure is that a pulse value and time of feeding it to a roll do not correlate with time of crossing longitudinal reinforcement elements by transverse reinforcement elements, since the device description lacks a means for carrying out synchronous processes. The result is low strength of a pipe both in the axial and the radial directions.
Furthermore, as a drawback of the prototype, it may be said that the mandrel is arranged directly after the extrusion passage. A melt leaving the feeding passage of an extrusion head goes directly onto a cooled mandrel. A melt leaving the passage has a temperature that is higher than a polymer melting temperature (e.g., the melt temperature of polypropylene is app. 190-270° C.). A melt, which comes to the end and the rear part of a mandrel, transfers a part of its heat to the latter. In such a case, on one side, early cooling of the melt occurs, which causes the early beginning of crystallization process and adhesion of a polymer to a metal framework, which results in lowering the pipe cross-sectional strength. On the other side, the action of a melt high temperature on a mandrel at the time when the process technology provides for its cooling does not enable to control and adjust the polymer cooling process, determine accurately and correct the beginning point of its adhesion to the reinforcing framework and its crystallization. Consequences are drawbacks that arise in the polymer matrix structure of a pipe that consists of 70-90% crystallites (i.e., areas of high density) and of 10-30% amorphous areas (i.e., areas of chaotic molecular bonds or areas of low density). Such polymer structure is characterized by low flexibility. When significant radial and axial loads are applied to a pipe made of a polymer with such a structure, its cracking occurs; therefore, a pipe produced according to the prototype has low indices of long-term strength.
One more significant drawback of the prototype is the organization of welding process for a reinforcing framework. Welding is carried out with roll electrodes that are mounted in pairs only. A number of roll electrodes corresponds to a number of transverse reinforcement coils and is selected from the even numbers (2, 4, 6 . . . ). There is no possibility of selecting a number of reinforcing coils from the odd numbers, which narrows the range of construction possibilities when making a pipe.
A pneumatic cylinder is provided for in the structure of the welding mechanism, which cylinder exerts a force necessary for pressing longitudinal reinforcement elements to transverse reinforcement elements. The lever of one roll electrode is secured to the pneumatic cylinder body, the lever of another roll electrode is secured to the rod; these levers form a mutually connected pair. When the air is forced into the pneumatic cylinder, a distance between the axes of securing the levers to the rod and to the body of the pneumatic cylinder increases. Pressing uniformity depends mainly on correct arrangement of rotation supports of the lever mechanisms. If their correct arrangement is not achieved, then geometric characteristics of movements of the lever mechanisms, and, consequently, roll electrodes will be different. Differences in the geometric characteristics of the lever mechanism movements will have an effect on pressing quality of each individual roll to a coil. It directly leads to a difference in the directions of force vectors relative to the axis of symmetry in plan. If forces applied to the lever supports on the body and on the rod of the pneumatic cylinder are equal, but angles between the pressing vector of a roll and the axis of symmetry are different, different pressing forces arise. As the result, a product made according to the prototype is a metal-polymeric reinforced pipe having a reinforcing framework with periodically changing strength and quality of welding connections between reinforcing coils and longitudinal reinforcement.
The claimed invention is aimed at eliminating the above-described drawbacks during development of a method for continuous making of a metal-polymeric reinforced pipe and a device for carrying out same and at guaranteed production of high-quality metal-polymeric reinforced pipes.
A pipeline is known in the art that is composed of metal-polymeric reinforced pipes and is described in Patent No. EP 1577077 dated Sep. 21, 2005, wherein pipes are connected with the use of electric-welded couplings. The use of electric-welded couplings for each connection makes the pipeline construction unnecessarily expensive; moreover, pipe connections are made permanent, which lowers workability of a pipeline, makes repair works more difficult as well as does not ensure the possibility of making connections between polymeric and metal pipes.
Several patents owned by Zapsibgasprom Ltd teach connection of plastic reinforced pipes by butt welding with subsequent tying of flange couplings. Couplings are provided with thread and have the possibility of moving axially. The said couplings are metal. (See: Patents Nos. RU2202727, RU2217311, RU33634). Differences in linear thermal expansion coefficients of a material of a metal coupling and a polymeric material of the matrix of a plastic reinforced pipe lead to delamination of such connection including a metal coupling and a metal-polymeric pipe at a change in external temperature or a change in temperature of a product transported via a pipeline.
In order to connect two ends of a metal-plastic pipe more reliably, it is necessary to provide them with connecting elements, so-called edge couplings that are rigidly fixed on the pipe ends.
The closest analogous solution for a pipeline connecting element is disclosed in the specification to Patent No. RU2085383 (published on Jul. 27, 1997) granted for “Method for radial friction welding of tubular parts based on polyolefines”. The pulling to the said patent shows a polymeric edge part with a protrusion of the back side, which protrusion closes reinforcement outlets on a pipe. The edge part has thread on the external surface, but has no thread on the internal surface, since is connected to a pipe by friction welding.
An edge part is mounted onto a fixed pipe with prepared contact surface by reciprocal movements. This method provides a connection having necessary strength and tightness.
The drawbacks of this connection are high labor-intensity and lack of possibility of using it in the field, directly on a pipeline, without dismounting and transportation to a production site.
The proposed connecting element and pipeline have no drawbacks described above.